I've written some prolog so:

<!DOCTYPE TEI.2 PUBLIC "-//TEI P4//DTD Main Document Type//EN" "some.dtd" [
<!NOTATION PNG SYSTEM "http://www.w3.org/TR/REC-png">
<!ENTITY some.png SYSTEM "some.png" NDATA PNG>]
>
Trying to parse this with psgml, I always come up with

Illegal entity type: NDATA

However, nsgmls validates the file. I also notice that there are other problems with ENTITIES and psgml. Is this a bug?